Gas turbines are used globally for power generation and various process applications. A gas turbine generally includes a compressor section, a combustion section, a turbine section, and an exhaust section. The compressor section progressively increases the pressure of a working fluid entering the gas turbine and supplies this compressed working fluid to the combustion section. The compressed working fluid and a fuel (e.g., natural gas) mix within the combustion section and burn in a combustion chamber to generate high pressure and high temperature combustion gases. The combustion gases flow from the combustion section into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator, thereby producing electricity. The combustion gases then exit the gas turbine via the exhaust section.
One of the primary advantages of a gas turbine is the ability to run on a variety of fuels. For example, gas turbines may burn heavy fuel oils, naphtha, distillate, flare gas, syngas, landfill gas, and natural gas. This is particularly advantageous in parts of the world that suffer from normal and seasonal shortages of various fuels or have an abundance of multiple different fuel types. As a result, many power plant owners operate gas turbines capable of burning multiple fuel combinations. For example, some gas turbines burn natural gas as a primary fuel and diesel or distillate as a backup fuel. Preferably, the gas turbine is able to automatically transfer between fuel types.
In order to facilitate automatic fuel transfer, some gas turbines include methods and systems implementable on short notice to facilitate a fuel change (e.g., from natural gas to diesel) while maintaining a high percentage of base power output. The standard approach rapidly reduces the load on the gas turbine to some predetermined value based on a valid determination that there will be a loss of or substantial reduction in natural gas pressure, while simultaneously transferring to and ramping up the backup fuel supply. This process requires purging and prefilling of the backup system and reducing the primary fuel flow by closing the main supply valve or gas control valve (GCV).
The success of the traditional response is dependent on the rate of decay of the primary fuel pressure and the time to vent and prime the backup fuel supply system. Rapid activation and transition to the full operation of the backup fuel supply system strains the gas turbine components and control system. The need to maintain power output while the gas turbine experiences fuel system, combustion system, and power generation transients further complicates the process.
Although known transfer processes are generally successful, system reliability issues may arise. In systems using diesel as the backup fuel, residual liquid fuel in the supply lines hardens into coke deposits after switching back to gaseous fuel and operating continuously thereon for about five days. These coke deposits may reduce overall supply line diameter. Additionally, coke formation hinders the proper functioning of spring-operated check valves and three-way valves and may even clog critical fuel system components (e.g., distributor valves, fuel flow check valves, fuel lines, etc.). Furthermore, coke formation in the liquid fuel lines leading to the combustion cans may reduce the probability of a successful transfer back to liquid fuel.
The prior art generally includes two type systems that seek to mitigate the coking problem, but each system has drawbacks. Liquid fuel recirculating systems continually recirculate liquid fuel back to a supply tank, thereby preventing fuel stagnation in a portion of the supply lines. But, the recirculation occurs upstream of the distribution valves, check valves, and other components. As such, these components may still suffer from coking Nitrogen purge systems use nitrogen gas to blow out residual liquid fuel in the supply lines. Although these types of systems are generally effective, they require that the gas turbine stop for the liquid fuel system purge before operating on gaseous fuel, thereby impacting plant availability. Furthermore, both types of prior art systems are capital intensive and may negatively impact production efficiency throughout the operating year. Moreover, both systems may be ineffective in areas of the world where gaseous fuel supplies are sporadic or where no additional downtime to clean system components after a failed transfer or restart is available.
Accordingly, a method and system for permitting a gas turbine to quickly transition from operating on gaseous fuel to liquid fuel and for purging the liquid fuel supply lines after transitioning back to gaseous fuel would be useful in the art.